1. Field of the Invention
The present invention relates to a substrate processing method and a substrate processing apparatus, and in particular to a method of processing a substrate having a hard mask comprised of a silicon oxide film and/or a silicon nitride film.
2. Description of the Related Art
In the case of forming a trench or a hole in a silicon (Si) base material 40 of a semiconductor wafer (hereinafter referred to merely as a “wafer”) W as a substrate that has the silicon base material 40 and a silicon nitride (SiN) film 41 formed on the silicon base material 40, the wafer W is subjected to dry etching using plasma 44 of halogenated gas such as hydrogen bromide (HBr).
In the dry etching, a hard mask 42 that is a film made of BSG (boron silicate glass) formed on the silicon nitride film 41 is used. The hard mask 42 has an opening 43 to which the silicon base material 40 is partially exposed. The plasma 44 enters the opening 43 and contacts the silicon base material 40 to physically or chemically etch the silicon base material 40 (FIG. 4A). As a result, a trench 45 corresponding to the opening 43 is formed in the silicon base material 40 (FIG. 4B).
However, during the dry etching described above, SiOBr produced through reaction of silicon and plasma of hydrogen bromide accumulates on a side face of the opening 43 and a side face of the trench 45 to form an SiOBr deposit portion 46. In particular, the amount of SiOBr accumulated on the side face of the opening 43 is large, and hence the SiOBr deposit portion 46 blocks most of the opening 43 and inhibits the entry of the plasma 44 into the trench 45. As a result, the etching of the silicon base material 40 stops, making it impossible to further deepen the trench 45.
To cope with this, as an example of a conventional method of removing the SiOBr deposit portion 46, a method in which the SiOBr deposit portion 46 is removed through wet etching using hydrofluoric acid has been developed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2002-33313). After the SiOBr deposit portion 46 is removed, the silicon base material 40 is etched again by the plasma 44.
However, in the wet etching using hydrofluoric acid, not only the SiOBr deposit portion 46 but also the hard mask 42 is removed. When the hard mask 42 is removed, part of the silicon base material 40 which does not correspond to the opening 43 becomes exposed, and the exposed part of the silicon base material 40 is etched, too, and hence the trench 45 becomes deformed, making it difficult to realize a desired aspect ratio of the trench 45.